[2013/02/22 19:06:37.373564, 0] param/loadparm.c:8004(lp_do_parameter)
Global parameter guest account found in service section!
This is annoying but the solution is simple:
The line "guest account = " is in the section for a share (in /etc/samba/smb.conf), but it should be under globals. Move it under [global] and then everything should be fine.